Class
One of the most evident and persistent themes in the film is class. At the start of the film, we are introduced to the Kims, a family struggling to live under impoverished conditions. When they get jobs working for the wealthy Park family, one-by-one, they see just how easy some people have it, and how much more luxuriously some people live. Throughout the film, they covet the lives of their employers, and get caught up in a struggle with Moon-gwang and her husband, who are also trying to benefit from the wealth of the Park family.
Wealth is depicted in many different lights throughout the film. It's shown as something that makes one brighter, more natural, nicer, but also something that dulls one's sense of accountability and meaning, that puts one "out of touch." The Parks are kind, but also shallow, helpless, and naive in many ways. The security of wealth remains the dream of the Kim family for the entirety of the film, with Ki-woo dreaming of buying the Parks' old house at the end of the film. However, the film also seeks to show the ways that wealth and class inequality is what creates the violence that bursts out in the final act.
Parasitism
The title suggests that the characters are parasites of some kind, entities that feed off a larger host. In interviews, director Bong Joon-ho has suggested that while the most obvious representations of parasites are the poor characters in the film—both the Kim family as well as Moon-gwang and Geun-sae—he also suggests that one might just as easily say that the Park family are the parasitical entity, in that they are completely helpless and feed off the skills and obedience of their employees.
Finally, one might also say that the central parasite in the film is class inequality itself. The aspirational wealth of the Parks is a kind of parasite of competition, a nagging sense of "never enough." Likewise, the poorer characters share this parasite, the desire to become upwardly mobile and enjoy all the benefits of wealth.
Violence
Violence emerges slowly in the film, as a companion to class inequality and desperation. Firstly, there is the psychological and political violence that the Park family enacts on the Kim family, inviting them into their lives, but also excluding them in ways that make clear their inequality.
Then, when the Kims are staying at the Parks' house, Chung-sook teases Ki-taek, calling him a "cockroach," and playing at the kind of structural violence that they face as poor people. Then, the struggle that breaks out between Moon-gwang, her husband, and the Kims is a perverse violence, one in which each party is fighting to maintain their relation to the Parks' wealth, in hopes that it can protect them from the hardships of life.
We see many different kinds of violence in the film, but none so gruesome as that of the final scenes. Chung-sook pushes Moon-gwang down the stairs. Geun-sae bludgeons Ki-woo with his rock, then stabs Ki-jung. Chung-sook stabs Geun-sae with a meat skewer to protect herself. Ki-taek stabs Mr. Park in a rage, after seeing his boss behave condescendingly towards the smell of Geun-sae. The end of the film is a bloodbath, when all the implicit violence of the structural and political positions of the characters comes brutally out into the open.
Deception
A great deal of the humor and pleasure of the film is watching the Kim family pull one over on the unsuspecting Parks. After Ki-woo gets hired as a tutor, he sets each of his family members up with a job, one by one, in the Park household. While the Parks do not know it, not only are all of their employees' qualifications fabricated, but they are all part of the same family. This creates a highly humorous and suspenseful dramatic irony, in which the viewer watches the Kims figure out how to keep their plot a secret, even in the most unthinkable circumstances. For instance, when the Parks return home from their camping trip unexpectedly, they wander through their home, blithely unaware of the fact that Moon-gwang and her husband are trapped in a bunker beneath the house, or that Ki-jung, Ki-woo, and Ki-taek are all lying underneath the coffee table in the living room.
Hope
Something that keeps the Kim family going is their sense of hope. They are tired of not being able to make ends meet, and are cynical about their class position in many ways, but they also hold onto a hope that their lives will improve, and it is this hope that keeps them going, even when they must lie in order to secure work.
Then, at the end, we see the theme of hope embodied in Ki-woo and Ki-taek, who correspond with one another while Ki-taek is squatting in the basement bunker after having killed Mr. Park. Ki-taek keeps himself alive, living the life of a scavenger, because he has the hope that his luck might change someday. Ki-woo hopes to one day be wealthy enough to purchase the house, and we see his fantasy on the screen. He enters the house, and Ki-taek simply wanders up the stairs, reunited with Chung-sook and his son. The final image of the film is Ki-woo penning this letter of hope to his father, imagining a better life for his family, even after all they have endured.
Environmental and Infrastructural Collapse
Bong Joon-ho stages scenes in which not only do poor characters have a lower quality of life, but their entire existences are in peril. For instance, on the night of the rainstorm, the Parks must return home and cancel their camping trip, but Da-song is able to spend the night in an adequately waterproof tent in the backyard. Contrastingly, when the Kims return to their basement apartment, their entire street is flooded with sewage water and their apartment is completely unlivable. Bong shows the way that being lower-class is not simply a matter of not having the same luxuries enjoyed by the upper classes, but more significantly, a matter of greater vulnerability to the elements, to disease, to environmental and infrastructural collapse.
Capitalism and Colonialism
The class struggle that Bong Joon-ho depicts is one that is directly connected to the inequalities built into a capitalistic system. Mr. Park is a well-educated businessman who has been afforded many more opportunities than any member of the Kim family. His opportunities are the result of his already having come from a wealthy family. The pit that the Kims find themselves in, in which they are unable to secure employment that can support them, is not a reflection of their abilities, but of their position within a capitalist economy.
Sewn into this depiction of capitalism is the suggestion of colonialism. The Parks feel entitled to take up more space than the Kims, which is in itself a kind of colonial impulse. The story can be seen as an allegory for the colonialism that is inherent to capitalistic growth. This theme is even hinted at when young Da-song and his father Mr. Park are wearing Native American headdresses at the party when the violence breaks out.
